JP2010131617A - Method of manufacturing ultra-thin wall seamless metallic pipe using floating plug - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To propose a method of manufacturing an ultra-thin wall seamless metallic pipe by cold drawing method by which the manufacturable range on the thin-wall side of the metallic pipe can drastically be extended. <P>SOLUTION: In this expanding/drawing method, an inlet-side solid die the diameter of which is expanded, constant or reduced toward the outlet side of finishing from the inlet side of biting and an outlet-side solid die the diameter of which is reduced at least in the vicinity of the inlet of biting are connected to each other and arranged oppositely and a taper type plug the diameter of which is expanded toward the outlet side of the finishing from the inlet side of biting is floated and, in the range of the inlet-side solid die, the wall thickness is reduced while expanding the diameter of the thickness center of a pipe stock between the solid die and the plug and, in the range of the outlet-side solid die, the taper type plug is floated by reducing the outside diameter. Since the manufacture of the ultra-thin wall seamless metallic pipe and long-size pipe making are made possible by the method and rationalization of a process is attained in the respects of yield, efficiency or the like, economical merit is remarkably large. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a cold drawing method for ultra-thin seamless metal pipes, adopting a diameter expansion drawing method using a floating plug, and in particular, dramatically expanding the manufacturable range on the thin wall side of metal pipes. Accordingly, the present invention intends to provide an efficient manufacturing method for an ultra-thin seamless metal pipe.

  The metal tube is sent to the cold working process if it does not satisfy the requirements of quality, strength or dimensional accuracy in the hot finished state. The cold working process includes a cold drawing method using a solid die and a plug or a mandrel, and a cold rolling method using a cold pilga mill, and the present invention relates to a cold drawing method.

  In the cold drawing method, a pipe end of a raw pipe is drawn with a mouth-drawing machine, and surface scales and the like are removed by pickling, and then a lubrication treatment is performed and drawing is performed through a die. Cold drawing methods include plug drawing, floating plug drawing, mandrel drawing, and empty drawing, all of which are performed by diameter reduction using a die.

  FIG. 1 is a diagram for explaining a conventional diameter reduction drawing method and shows a case of plug drawing. The plug drawing shown in the figure is the most common drawing method. The plug 3 supported by the plug support rod 4 is inserted into the raw tube 1, the tube end of the raw tube 1 is held by the chuck 5, and the die 2 is inserted. And drawing in the direction of the arrow indicated by the symbol X in the figure. This method is excellent in plug replacement and workability, and has a high degree of processing.

  Further, although illustration is omitted for the case of mandrel pulling, it is a method of inserting a mandrel into a blank tube and pulling it out through a die in the same manner as shown in FIG. In this method, since the inner surface of the tube is processed by a mandrel, a product having a beautiful inner surface and high dimensional accuracy can be manufactured even if it is a small diameter tube, and is used for manufacturing a high-grade tube for nuclear power.

  As a drawing machine used for cold drawing, a chain type driven by a motor is widely adopted, but a hydraulic type or a hydraulic type is also used.

  In the cold drawing process of metal tubes, there is a frictional resistance between the outer surface of the tube material and the die surface, and between the inner surface of the tube material and the plug or mandrel surface. Tension occurs in the longitudinal direction. When the tension stress obtained by dividing this tension by the cross-sectional area after drawing becomes high, thinning starts, and when the tensile stress reaches the deformation resistance of the tube material, it breaks.

  Since the tensile stress in the longitudinal direction increases as the thickness of the tube becomes thinner and breaks easily, the rate of reduction in thickness is naturally limited. Therefore, in the drawing process in which the thickness reduction rate becomes large, it is necessary to repeat the drawing operation by increasing the number of drawing operations, and a lubrication operation is required each time, resulting in an increase in cost. Further, when the work hardening of the pipe material is remarkable, an annealing operation is also required between repeated drawing processes.

PCT / JP2008 / 051619 application specification

  The present invention has been made in view of the above-mentioned problems, and the problem is to propose a method for manufacturing an ultra-thin metal tube by the cold drawing method that can dramatically expand the manufacturable range of the metal tube on the thin wall side. There is to do. Although the present invention is mainly intended for thin-walled seamless metal pipes, even in thin-walled welded metal pipes, unevenness in the thickness at the welded part or heat-affected zone may occur, and correction thereof may be necessary. Therefore, weld metal pipes are also included in the target.

  In order to solve the above-mentioned problems, the present inventor has advanced research and development based on conventional problems, obtained the following knowledge, and invented a method for producing an ultra-thin metal tube by cold drawing, It has already been proposed as Patent Document 1.

  Generally, the wall thickness processing in the plastic processing of a pipe material is achieved by drawing a pipe material in the longitudinal direction of the pipe. That is, in the cold drawing of the pipe material, when the wall thickness processing is performed between the die and the plug or the mandrel, the pipe material is drawn while being reduced in diameter and is drawn in the longitudinal direction.

  On the other hand, the present inventor tries to stretch only in the longitudinal direction when plastically processing the pipe material to reduce the wall thickness, so that the amount of wall thickness reduction is limited, making further thinning difficult. Therefore, it was considered that the above-mentioned problem can be avoided by extending the pipe material in the longitudinal direction at the same time as the pipe wall thickness is reduced by the cold drawing method.

  Incidentally, as an extreme case, when considering the rolling of an annular product by a ring mill, the annular material is not stretched in the longitudinal direction (axial direction) but is stretched only in the circumferential direction. It becomes possible without limit.

  In the drawing process, in order to extend in the longitudinal direction while extending in the pipe circumferential direction, the drawing may be performed while expanding the diameter with a plug or a mandrel. If at least a plug or a mandrel having an inner surface regulating diameter larger than the outer diameter of the raw tube is used, the diameter of the raw tube can be reliably expanded.

  As described above, if the raw tube is drawn while expanding the diameter, the circumferential length increases even if the wall thickness is reduced, so the cross section of the pipe material does not decrease so much, and the tensile stress during drawing is reduced. There is also an advantage that can be reduced.

  The invention disclosed in Patent Document 1 (hereinafter also referred to as “prior application invention”) has been completed based on the above findings, and the gist thereof is the cold shown in the following (a) to (d) It is a manufacturing method of an ultra-thin metal tube by a drawing method.

  (A) An ultra-thin metal tube manufacturing method using a drawing machine, in which a diameter of the tube gradually expands or decreases from the biting inlet side toward the finishing outlet side, and squeezed at one end of the pipe Insert a plug or taper mandrel that gradually increases in diameter from the die biting inlet side to the finishing outlet side, and grip the part that has been widened with a chuck. By pulling out the raw pipe in the direction from the biting inlet side to the finishing outlet side, the wall thickness center diameter, which is the average of the outer diameter and inner diameter of the pipe material, is increased between the solid die and the plug or taper mandrel. A method for producing an ultra-thin metal tube by cold drawing, characterized in that the thickness is reduced while being drawn.

  (B) Stretching while reducing the wall thickness while expanding the inner diameter and the outer diameter at the same time while taking the expansion allowance for the inner diameter larger than the expansion allowance for the outer diameter. Manufacturing method of ultra-thin metal tube by cold drawing method.

  (C) The method for producing an ultra-thin metal tube by the cold drawing method according to (a), wherein the outer diameter is kept constant and the thickness is reduced while expanding only the inner diameter.

  (D) The above-mentioned (a), characterized in that the outer diameter is reduced while the inner diameter expansion allowance is larger than the outer diameter reduction allowance, and the inner diameter is increased while the thickness is reduced while the inner diameter is increased. A method for producing an ultrathin metal tube by the cold rolling method described in 1.

  FIG. 2 is a diagram showing a diameter expansion drawing method in the case of plug pulling in the above-mentioned prior application invention. FIG. 5A shows a case where drawing is performed while simultaneously expanding the inner diameter and the outer diameter, FIG. 5B is a case where drawing is performed while only the inner diameter is expanded while the outer diameter is constant, and FIG. Indicates a case where the diameter is reduced and the drawing is performed while the inner diameter is increased.

  As shown in FIGS. 4A to 4C, the diameter of the die is increased from the biting inlet side of the die (left side of the solid die 21 in the drawing) toward the finishing outlet side (right side of the solid die 21 in the drawing). From the finishing exit side of the solid die 21 to be expanded, the raw tube 1 subjected to the opening expansion is inserted into the solid die 21.

  Further, a plug 31 whose diameter is enlarged from the inlet side to the outlet side of the solid die 21 and whose maximum finished diameter is larger than the outer diameter of the raw pipe 1 is inserted into the raw pipe 1 and the raw pipe is subjected to widening. 1 is gripped by the chuck 5 and pulled out in the direction of the arrow indicated by the symbol X in the figure. By this operation, the raw tube 1 is drawn while the wall thickness center diameter is increased between the plug 31 and the solid die 21. The mandrel pulling was omitted.

  As described above, the invention for the ultra-thin seamless metal tube has been obtained by the invention of the prior application. However, in the invention of the prior application, the structure in which the tapered plug such as the plug 31 is supported by the plug support rod 4 is used. It is necessary to use a drawing machine. Therefore, the length of the raw tube 1 is restricted, and it is difficult to perform drawing of the long tube, and the yield loss in the squeezing process and the squeezing process part of the pipe cannot be ignored. In order to break through this problem, the present inventor decided to challenge the development of a diameter expansion drawing method using a floating plug.

  The present invention relates to an expanded cold drawing method using a floating plug, that is, a floating plug. Around 1964, the cold drawing method using floating plugs developed and put into practical use (however, diameter reduction) is the most innovative technology in the history of 170 years of cold drawing of seamless metal tubes. It was an innovation. As a result, the long tube can be drawn, and the coil form by the bull block drawing method can also be realized.

  In the cold drawing method using a floating plug, a tapered plug that gradually reduces its diameter from the inlet side to the outlet side is the same as a solid die that gradually reduces its diameter from the biting inlet side to the finishing outlet side. Use cold drawing. In this case, the reason why the plug can be floated is that the plug is supported by the balance of the force acting on the plug (the pulling force and the retreating force) on the exit side of the reduced diameter die, and is in a floating state. . On the other hand, if a diameter-expanding die and a diameter-expanded plug are used to expand the diameter, the plug cannot be completely floated.

  In order to solve this problem, the present inventor has placed two solid dies on the pass line so as to face each other, and floats a diameter expansion plug on the inside to thereby perform diameter expansion drawing. It was found that it was possible to do.

  That is, in the cold drawing method of an ultra-thin seamless metal pipe using a floating plug, the diameter of the entrance side solid die is increased, fixed, or reduced from the biting inlet side to the finishing outlet side. The outlet side solid die whose diameter is reduced at least in the vicinity of the biting inlet is connected and disposed on the same path line, and the diameter gradually increases from the biting inlet side to the finishing outlet side. The taper type plug is floated on the pass line, and in the entry side solid die region, the wall thickness is increased while increasing the wall thickness center diameter, which is the average value of the outer diameter and inner diameter of the raw tube between the solid die and the plug. The taper-type plug is floated in the vicinity of the inlet of the outlet solid die, and the diameter-expanded drawing can be performed. In this case, a slight thickness processing may be performed by the outlet side solid die.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in a method for producing an ultrathin seamless metal tube by diameter expansion drawing shown in the following (1) to (4).

  (1) Cold drawing method for ultra-thin seamless metal pipes using a floating plug, the diameter of which increases from the biting inlet side toward the finishing outlet side, and is constant or reduced. A solid die and an outlet side solid die whose diameter is reduced at least near the biting inlet are connected to each other on the same path line, and a raw pipe is placed in the inlet side solid die and the outlet side solid die. A tapered plug that is inserted and gradually increases in diameter from the biting inlet side to the finishing outlet side is floated on the pass line, and the tube end of the element pipe is gripped by the chuck from the inlet side to the outlet side. By pulling out the pipe in the direction of the direction, in the entry side solid die region, the wall thickness center that is the average value of the outer diameter and inner diameter of the pipe between the solid die and the plug While expanding the reduced wall thickness, in the exit-side solid die region, the production method of the ultra thin wall seamless metallic tube by diameter expansion drawing for causing floating the tapered plug by performing the rolling of the outer diameter.

  (2) In the entrance-side solid die region, the inner diameter expansion allowance is made larger than the outer diameter enlargement allowance, and the inner diameter and the outer diameter are simultaneously expanded while extending the thickness to reduce the thickness. The manufacturing method of the ultra-thin seamless metal pipe by the diameter expansion drawing as described in said (1).

  (3) Ultra-thin wall due to diameter expansion drawing as described in (1) above, wherein, in the entry side solid die region, the outer diameter is kept constant and the thickness is reduced while expanding only the inner diameter. A method for producing seamless metal pipes.

  (4) In the entry-side solid die region, the outer diameter is reduced, and the inner diameter is increased while the inner diameter is increased and the wall thickness is reduced, and the ultrathin joint by the diameter expansion drawing according to (1) above A method for producing metal-free tubes.

  In the present invention, the “ultra-thin seamless metal tube” means a seamless metal tube having a ratio t / d between the wall thickness t and the outer diameter d of 4.0% or less. Here, the seamless metal pipe includes a welded metal pipe as described above.

  According to the method of the present invention, an entrance-side solid die whose diameter increases from the biting inlet side toward the finishing outlet side, which is constant or reduced, and an outlet side whose diameter decreases at least near the biting inlet A solid die is connected on the same pass line and arranged oppositely, and a taper type plug whose diameter gradually increases from the biting inlet side to the finishing outlet side floats on the pass line, and enters the solid side. In the die area, reduce the wall thickness while expanding the wall thickness center diameter, which is the average value of the outer diameter and inner diameter of the raw tube between the solid die and the plug, and reduce the outer diameter in the outlet solid die area. Since the taper-type plug is floated, the production range on the thin wall side of the seamless metal pipe by the cold drawing method can be dramatically expanded. In addition, since a long tube can be manufactured, the process can be rationalized in all aspects such as yield and efficiency.

  As described above, the present invention is a cold drawing method of an ultra-thin seamless metal pipe using a floating plug, and its diameter increases from the biting inlet side toward the finishing outlet side, and is constant. Alternatively, the inlet solid die to be reduced and the outlet solid die whose diameter is reduced at least in the vicinity of the biting inlet are connected on the same path line so as to face each other. The pipe is inserted into the pipe, and a taper type plug whose diameter gradually increases from the biting inlet side to the finishing outlet side is floated on the pass line, and the pipe end of the pipe is grasped by the chuck. By pulling out the pipe in the direction from the side to the outlet side, the outer diameter and inner diameter of the pipe between the solid die and the plug are mainly in the entry side solid die region. Ultra-thin by expansion drawing, characterized by reducing the wall thickness while increasing the wall thickness center diameter, which is the average value of the above, and reducing the outside diameter to float the tapered plug in the outlet solid die region This is a method for manufacturing a seamless metal pipe. Below, the manufacturing method of the ultra-thin seamless metal pipe of this invention is further demonstrated using drawing.

  For comparison, FIG. 3 shows a diagram for explaining a diameter reduction drawing method using a conventional floating plug. A taper type plug 32 whose diameter gradually decreases from the biting inlet side to the outlet side is inserted into the raw pipe 1, and the pipe end of the raw pipe 1 is floated while the taper type plug 32 floats on the pass line. Is drawn by a chuck 5 and pulled through a die 2 whose diameter gradually decreases from the biting inlet side toward the outlet side in the direction indicated by symbol X in the figure.

  However, as described above, even if the diameter expansion drawing is performed using the diameter expansion die and the diameter expansion plug by this method, the plug cannot be floated.

  FIG. 4 is a diagram for explaining a form of the diameter expansion drawing method using the floating plug according to the present invention. (A) shows a form of drawing while simultaneously expanding the inner and outer diameters in the entrance-side solid die region, and (b) shows only the inner diameter being expanded while keeping the outer diameter constant in the entrance-side solid die region. (C) shows a form in which the outer diameter is reduced in the entry-side solid die region and the drawing is performed while the inner diameter is increased.

  In the form shown in FIG. 5A, the entrance-side solid die 22 whose diameter gradually increases from the biting inlet side toward the finishing outlet side, and the outlet-side solid die 23 whose diameter decreases near the biting inlet. Are connected to each other on the same pass line 6. Then, the raw tube 1 having the outer diameter do and the wall thickness to is inserted into the entrance-side solid die 22 and the exit-side solid die 23, and the diameter gradually increases from the biting inlet side toward the finishing outlet side. The tapered plug 33 is floated on the pass line 6, the tube end of the element tube 1 is gripped by the chuck 5, and the element tube 1 is pulled out in the direction indicated by the symbol X in FIG.

  The floating plug according to the present invention includes a tapered portion 33a, a bearing portion 33b, and a floating portion 33c. In the entrance-side solid die 22 region, the thickness is increased while increasing the thickness center diameter, which is the average value of the outer diameter and inner diameter of the tube 1 between the solid die 22 and the taper portion 33a and the bearing portion 33b of the plug. Decrease. In the region of the outlet side solid die 23, the outer diameter is reduced, the taper type plug 33 is floated by the action of the floating portion 33c of the plug, and the ultra-thin seamless with the outer diameter d and the wall thickness t is obtained by the diameter expansion drawing. Metal tubes can be manufactured.

  In the form shown in FIG. 6B, the outer diameter of the raw tube 1 is kept constant in the region of the entrance-side solid die 22, and the thickness is reduced while expanding only the inner diameter. Further, in the form shown in FIG. 5C, in the entrance-side solid die 22 region, the outer diameter is reduced, and the inner diameter is increased while the inner diameter is increased while the wall thickness is reduced.

  In any form, the drawing is performed in the entrance solid die 22 region while increasing the thickness center diameter by the taper portion 33a and the bearing portion 33b of the plug. At the same time, the tapered plug 33 is floated by the action of the plug floating portion 33c by reducing the outer diameter in the region of the outlet side solid die 23.

  FIG. 5 is a diagram showing a configuration example of pre-processing for performing cold drawing according to the present invention. In carrying out the present invention, there are various pre-treatments for placing two solid dies on the pass line so as to face each other and floating the tapered plug inside thereof, and FIG. Yes.

  In FIG. 5 (a), the known tube (for example, the drawing method shown in FIG. 2) is used to insert the blank tube 1 into the inlet solid die 22 from the biting inlet side, and to insert the tapered plug 33 into the blank. The tube 1 is inserted into the tube 1 and drawn while expanding the diameter between the inlet solid die 22 and the taper type plug 33. At this time, the taper type plug 33 needs to hold the output side end portion thereof for drawing positioning (positioning of the finish portion of the entrance side solid die 22 and the bearing portion 33b of the taper type plug 33).

  In FIG. 5B, the reduction roller 7 is provided on the outlet side of the taper type plug 33, and the outer peripheral portion of the raw tube 1 is pressed to reduce the diameter along the floating portion 33 c of the plug. At this time, the diameter reduction of the pipe is made smaller than the finishing dimension of the outlet side solid die 23.

  In FIG. 5 (c), the outlet solid die 23 is inserted from the outlet side of the diameter-reduced element tube 1, and is placed opposite to the inlet side solid die 22, and the taper type plug 33 is floated inside both the dies. . Thereby, the pre-processing for implementing the cold drawing of this invention is complete | finished.

  In order to confirm the effect of the manufacturing method of the ultra-thin seamless metal pipe by the diameter expansion drawing method using the floating plug according to the present invention, the following test was performed and the result was evaluated. The test was performed by the method shown in FIG. 4 (a). However, the method shown in FIGS. 4 (b) and (c) is almost the same as the operation and effect of the method shown in FIG. In the example, the test results obtained by the method shown in FIG.

  An 18% Cr-8% Ni stainless steel pipe with an outer diameter of 48.6 mm, an inner diameter of 41.6 mm, and a wall thickness of 3.5 mm manufactured by the Mannesmann mandrel mill process is used as the test element pipe, The outer diameter is 53.8mm, the inner diameter is 50.8mm, the wall thickness is reduced to 1.5mm, and the wall thickness is reduced. The outer diameter is reduced near the entrance of the outlet solid die, the outer diameter is 50.8mm, the inner diameter is 47.8mm. Finished to a wall thickness of 1.5 mm.

The test conditions and results are summarized below.
Maximum diameter of tapered plug: dp = 50.8mm
Elementary tube dimensions Elementary tube outer diameter: do = 48.6mm
Base tube inner diameter: d′ o = 41.6 mm
Tube thickness: to = 3.5mm
Intermediate dimensions at the inlet side solid die outlet Outer diameter: 53.8mm
Inner diameter: 50.8mm
Thickness: 1.5mm
Product dimensions at outlet of outlet solid die Product outer diameter: d = 50.8mm
Product inner diameter: d ′ = 47.8 mm
Product thickness: t = 1.5mm
Expansion ratio: d / do = 1.05
Stretch ratio: to (do-to) / {t (dt)} = 2.13
(Thickness / outer diameter) ratio: t / d = 2.95%

  The inner and outer skins of the products obtained by the above test were beautiful, and there was no particular problem in quality. In addition, the minimum thickness of the austenitic stainless steel pipe by the conventional diameter reduction drawing method is about 2.4 mm when the outer diameter is 50.8 mm, and it is seamless by the diameter expansion drawing method using the floating plug according to the present invention. It is clear that the effect of the metal tube manufacturing method is remarkable.

  According to the diameter expansion drawing method using the floating plug according to the present invention, the manufacturable range on the thin wall side of the seamless metal pipe by the cold drawing method can be dramatically expanded. If it becomes possible to economically and stably produce a seamless metal pipe having a thickness of about 2/3 or less of that of the conventional reduced diameter drawing method by the expanded diameter drawing method, a TIG welded tube, laser welding, etc. An ultra-thin welded pipe such as a pipe can be substituted for the seamless pipe produced according to the present invention. In addition, since a long tube can be manufactured by adopting a floating plug, it is possible to rationalize the process in all aspects such as yield and efficiency.

It is explanatory drawing of the diameter reduction drawing method by the conventional plug pull. It is explanatory drawing of the diameter expansion drawing method by plug drawing, The figure (a) is a case where it draws while expanding an inner diameter and an outer diameter simultaneously, (b) expands only an inner diameter, with an outer diameter unchanged. (C) shows the case where the outer diameter is reduced and the drawing is performed while the inner diameter is increased. It is explanatory drawing of the diameter reduction drawing method using the conventional floating plug. It is explanatory drawing of the diameter-expanding drawing method using the floating plug which concerns on this invention, The figure (a) is a case where it draws, expanding an internal diameter and an outer diameter simultaneously in an entrance side solid-die area | region, (b) When the drawing is performed while the outer diameter remains unchanged in the entry-side solid die region while only the inner diameter is increased, (c) is drawn while the outer diameter is reduced in the entry-side solid die region and the inner diameter is increased. Each case is shown. It is a figure which shows the structural example of the pre-processing for implementing cold drawing of this invention.

Explanation of symbols

1: blank tube, 2: die, 3: plug,
4: Plug support rod, 5: Chuck,
6: Pass line, 7: Rolling roller,
21: Solid die, 22: Entrance solid die,
23: Outgoing solid die, 31: Plug,
32: Floating plug, 33: Floating plug,

Claims (4)

A cold drawing method of an ultra-thin seamless metal tube using a floating plug,
The same path line between the entrance solid die whose diameter increases, is constant, or contracts from the biting inlet side to the finishing outlet side, and the outlet solid die whose diameter decreases at least near the biting inlet Connected to the top and placed opposite to each other,
Inserting the raw pipe into the entry side solid die and the exit side solid die, and floating the taper type plug whose diameter gradually increases from the biting inlet side toward the finishing outlet side on the pass line, By grasping the pipe end of the pipe with a chuck and pulling out the pipe in the direction from the inlet side to the outlet side,
In the entry side solid die region, the wall thickness is reduced while increasing the wall thickness center diameter, which is the average value of the outer diameter and inner diameter of the raw tube between the solid die and the plug, and the outer diameter in the exit side solid die region. A method for producing an ultra-thin seamless metal pipe by diameter expansion drawing, wherein the taper type plug is floated by reducing the pressure.   2. In the entrance-side solid die region, the inner diameter expansion allowance is made larger than the outer diameter enlargement allowance, and the inner diameter and the outer diameter are simultaneously expanded while extending the thickness to reduce the thickness. The manufacturing method of an ultra-thin seamless metal pipe by diameter expansion drawing described in 1.   2. The ultrathin wall seamless metal pipe by diameter expansion drawing according to claim 1, wherein in the entrance-side solid die region, the outer diameter is constant and the thickness is reduced while expanding only the inner diameter. Manufacturing method.   2. The ultrathin wall seamless metal pipe by diameter expansion drawing according to claim 1, wherein in the entrance-side solid die region, the outer diameter is reduced and the thickness is reduced while the inner diameter is increased. Production method.

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